Several months ago, William L. Mayo was planning to retire as president of Comsat General Corp. and build a new home in the northern neck of Virginia.

He did retire from Comsat. But after all of one weekend in retirement, he was named president and chief executive officer of Questech Inc., a local defense contractor.

Mayo, 59, took over at McLean-based Questech on May 1, replacing Herbert W. Klotz, who died last November. John L. McLucas, Mayo's longtime associate at Comsat's parent, Communications Satellite Corp., had been acting as president, chief executive and chairman of Questech. McLucas remains chairman.

Questech is the holding company for four subsidiaries that specialize in national security, aeronautics, energy and industrial modernization. About 95 percent of its revenue comes from government contracts. But the company ran into financial difficulties last year, largely because of serious cost overruns on a government contract.

"{McLucas} called me up one day and asked me if I was interested in running Questech," Mayo said. "And, I said, 'Should I be?' " The two met that night to discuss possibilities for the company. McLucas knew Mayo planned to retire, although Mayo had not announced it.

Mayo first met McLucas when Mayo was vice president of international operations for GTE Corp. McLucas later recruited him to join Comsat in 1978 to lead the company's entrance into the commercial market. McLucas, former secretary of the Air Force, retired from Comsat as executive vice president in 1985 and was named a director of Questech the same year.

In Mayo's new office at Questech stands a larger-than-life replica of the original RCA radio dog, reminiscent of his days at Comsat. The president of NBC gave it to him after he signed a 10-year contract with NBC to broadcast to worldwide affiliates using satellites. The contract was estimated at $450 million.

Mayo also headed Comsat's joint venture with Holiday Corp. to provide movies, sports and news to Holiday Inns via satellite.

"It was the consensus that we needed someone from outside who knew how to manage large numbers of people and how to manage technology," said Vincent L. Salvatori, executive vice president of technology and planning. "There have been a lot of people holding their breath around here, but {Mayo's appointment} has had a positive affect on moral."

Questech had run into financial trouble in 1986 when its West Coast operation, Dynamics Engineering Inc.-West, suffered major losses due to cost overruns on a government hardware contract. The company has discontinued operations of the unit and took a $1.6 million write-off.

The company also has made "substantial investments" in one of DEI's technology groups and a joint venture with Rampart Packaging Co., a Shell Oil Co. subsidiary, to develop a plastic container that can substitute for the standard "tin" can.

Although the company's revenue rose 10.7 percent in 1986, net income was down sharply, from $857,006 (56 cents a share) in 1985, to $195,202 (13 cents) in 1986. However, for the first quarter of 1987, net income was $380,700 (25 cents), compared to a loss of $260,934 (17 cents) for the first quarter of 1985.

"The first-quarter results give a clear indication that they are on the upswing," said Eliott Benson, who follows Questech for Ferris & Co. "The elimination of the West Coast operations had taken away a lot of red ink."

Questech recently named former NASA executive David J. Peakes to head its DEI subsidiary and consolidated operations in Newport News, Va.

Benson called McLucas, Mayo and Peakes "highly respected businessmen. We believe they will provide good direction and control for the company."

Mayo said he plans to look at the company "with a fresh pair of eyes." He said that there is some overlapping in the subsidiaries and that there will be future consolidation. However, he said he has not yet developed a restructuring plan.

The company may look to commercial markets, Mayo said, but it would not be a major part of its thrust. The company plans to aggressively pursue government contracts instead of "growing with the defense budget," Mayo said. Backlog for government contracts was up 60 percent in 1986. BANKING, FINANCIAL SERVICES

Equitable Bank N.A. of Baltimore has appointed Jennifer Carey Bak and Nancy A. Smith vice presidents.

The Federal Home Loan Mortgage Corp. of the District has named Robert Van Order chief economist for the corporation. Van Order had been director of the Housing Finance Analysis division at the Department of Housing and Urban Development.

Chapin, Davis & Co. Inc., an investment firm in Baltimore, has named Walter A. Frey III president and chief operating officer and Alex G. Fisher chairman and chief operating officer. Frey joined the company from Legg Mason Wood Walker Inc., where he had been vice president and manager. Fisher, a four-year veteran of the company, joined the company from Mercantile-Safe Deposit and Trust Co., where he had been senior vice president of the trust personal services division.

Charles R. Clegg and Mary Lou Raymond have been named to the board of Farmers & Merchants National Bank.

Sovran Financial Corp. of Bethesda has promoted Ivy E. Barnett and George J. Appino to vice presidents. COMMUNICATIONS

Contel Corp. of Atlanta has appointed Stuart C. Johnson executive vice president of the company and president of its Federal Systems Sector headquartered in Fairfax. Johnson had been president of PRC Government Information Systems, a unit of Planning Research Corp. in McLean. He succeeds William Kamachaitis, who recently retired.

Kathleen Henderson-Whyte has joined Ketchum Public Relations of the District as vice president of government relations. She had been with Miles Laboratories Inc., a subsidiary of Bayer A.G.

Ellerbe Associates Inc. of the District has named Gregory Powe vice president of the corporate and commercial group. Powe had been regional president of Leason Pomeroy Associates of San Francisco.

Alternatives Inc. of the District has named Michael A. Glorioso vice president to oversee daily operations. Glorioso had been an account executive with the NBC-owned WKYS-FM radio station. HOTEL, FOOD SERVICES

Stephen Seeger has been named general manager of the Embassy Suites hotel in Crystal City and Debra Feldman Schwarz has been named general manager of the Embassy Suites at Tysons Corner.

Former vice president of marketing for the Madison Hotels Margaret Rauh has formed MWR Enterprises, a meeting and special events consulting firm.

Marriott Corp. has named Lori A. Cramp vice president of corporate finance. She joined the company in 1981 as a manager in the company's consulting department.

Stephen P. Peeck , former vice president of food and beverage for the Sheraton Corp., has been named managing director of Potomac Restaurant. INSURANCE

James J. Chastain has been elected president of the Washington chapter of Chartered Property Casualty Underwriters. He is a professor of insurance at Howard University.

Pension Benefit Guaranty Corp. of the District has appointed Joseph A. Vasquez Jr. deputy executive director and chief operating officer. Vasquez joined the company from the Office of Management and Budget, where he headed the central management and budget information and decision support systems.

David W. Berson, formerly of Wharton Econometric Forecasting Associates, has joined the staff of the economics department of the U.S. League of Savings Institutions. PROFESSIONAL

Harold D. Palmer , formerly executive director of the Marine Technology Society, has joined the senior scientific staff of MRJ Inc., an Oakton, Va. subsidiary of Perkin-Elmer Corp.

HTB Inc. of the District has named William R. Lawson vice president and director of the Washington office. Lawson had been assistant to the commissioner of the public building service for the General Service Administration.

Charles L. Heatherly, former acting administrator for the Small Business Administration, has been named vice president of academic relations at the District-based Heritage Foundation.

University Research Corp., a professional services company in Chevy Chase, has appointed Rosalyn E. Levy director of corporate communications. Levy had been the company's director of personnel, administrative and corporate services. UTILITIES

Constellation Holdings Inc., a Baltimore Gas & Electric Co. subsidiary, has formed three companies, Constellation Development Inc., Constellation Water Systems Inc. and Constellation Operating Systems Inc. Bruce M. Ambler has been named president of Constellation Development and Constellation Water Systems. He joined the company in December from Catalytic Inc., where he was senior vice president. Paul H. Steinbach was named president of Constellation Operating Systems.